UC Santa Barbara

Hydrogen is an attractive carrier of renewable energy due to its natural abundance, high energy density, and lack of harmful byproducts when converted to electricity in a fuel cell. Solid-state hydrogen fuel cells are gaining particular attention for their stability, storage capacity, and ability to operate at high temperatures. However, their technological adoption requires highly conductive solid-state hydrogen electrolytes.

In crystalline electrolytes, conductivity is tied to the concentration of mobile hydrogen defects, be they hydrogen interstitials or vacancies. Increasing the concentration of such defects improves the ionic conductivity. Understanding how to do so requires knowledge of the defect chemistry, including native point defects as well as extrinsic dopants and impurities.

To this end, we examine a suite of prospective solid-state hydrogen electrolyte materials using accurate first-principles calculations based on density functional theory with a hybrid functional. These include proton-conducting oxides, such as the alkaline-earth zirconates (CaZrO3, SrZrO3, BaZrO3) and cerates (SrCeO3 and BaCeO3), as well as several hydride-ion conductors, namely, the alkaline-earth hydrides (CaH2, SrH2, and BaH2), La-Sr-Li-H-O oxyhydrides, and the nitride hydride Sr2LiH2N. We report on calculations of defect formation energies under conditions representative of experimental growth and operation.

Our results suggest approaches for improving ionic conductivity through defect engineering. For instance, in the alkaline-earth zirconates and hydrides, we show that alkali metal dopants boost the concentration of mobile hydrogen defects, while also possessing low Coulombic binding energies to those defect species so as not to hinder their mobility. Native point defects and extrinsic impurities also affect these materials’ chemical stability. In the alkaline-earth cerates, carbon impurities hamper device performance and stability, and we show specifically why their impact is worse than in the zirconates. For the oxyhydrides and nitride hydride, the same defects that grant them high hydride conductivity provide a stabilizing effect that may permit synthesis.

The work summarized here lays a foundation for the development of solid-state fuel cells and of hydrogen as a reliable energy source. These goals require the engineering of novel materials such as those described here; thus, our results provide essential knowledge to help power a sustainable energy future.